link aggregation
termSeveral parallel Ethernet links grouped so that both ends treat them as one logical link, for more capacity and for resilience.
IEEE 802.1AX today, 802.3ad before it; EtherChannel or port channel at Cisco, aggregated Ethernet at Juniper, a bond on Linux, a trunk on F5. LACP is the protocol that negotiates it.
The standard's own definition is the point of it: links are 'aggregated together to form a Link Aggregation Group (LAG), such that a MAC Client can treat the LAG as if it were a single link'. Spanning tree sees one link and blocks nothing; if a member fails, the group keeps carrying traffic on the rest.
Two rules surprise people. Every member must match: Cisco asks for the same speed and duplex on all ports, and the same VLAN or trunk settings. And a single flow never runs faster than one member, because frames are spread by a deterministic hash of addresses and ports, so a conversation stays on one link to keep its packets in order; a group of four one-gigabit links serves many flows at four gigabits and one large transfer at one.
Build it with LACP rather than statically: a static group cannot notice a member that fails silently, and a mismatch between the two ends is, in Aruba's words, 'difficult to troubleshoot, because no signaling takes place between the two peers'.
Also known as: lag, link aggregation group, port channel, etherchannel, bond, bonding, 802.1ax, 802.3ad, aggregated ethernet